Human units can't be built. There is some natural natality and mortality that will affect your population. But generally if you want more villagers you will have to invade surrounding area, find villages and convice/buy/fight/force (depending on some attributes) their inhabitants to obey your orders.

Every being requires daily feeding. If it's starving its efficiency drops and it can get angry/desert/die. If it's villager returning every evening home it will be handled automatically. But if it's some scouting army unit he can carry food and water for 5 days and then has to resupply somehow.

Every villager has his set of attributes and specializations. If you use one villager for building more houses then his efficiency in building will increase.

How to make a cavalery unit. You will need:- one trained horse (you can catch it on plains or grow it in your pen and then train it, or you can buy it on global market, or steal it somewhere)- one trained villager (with army training and cavalery training - you can buy him or train him by your-self)- riding and fighting equipment (saddle, harness and sword - if you're making them, it will be taken from your supplies otherwise you have to buy/steal/find it somewhere)This can be fully automated - you will click cavalery unit and if it can be assembled you will get one, otherwise you will see the price and time required to create it and can order it. You can set you need 100 cavalery units and the game will slowly create them depending on your resources. Or you can specialize on horse training, offer them on market and make business with them.